Final answer:
Adaptive immunity is characterized by humoral immunity, where B cells produce antibodies against extracellular pathogens, and cellular immunity, where T cells target intracellular pathogens. Natural killer cells provide nonspecific defense as part of the innate immune system, and antigen-presenting cells help activate T and B cells.
Step-by-step explanation:
Adaptive immunity refers to the immune system's ability to specifically recognize and defend against distinct pathogens. This involves a dual system of protection through humoral immunity and cellular immunity.
Humoral immunity involves B cells (B lymphocytes), which are responsible for producing antibodies that are effective against extracellular pathogens. These antibodies bind to antigens and can neutralize or mark these pathogens for destruction.
In contrast, cellular immunity is primarily concerned with T cells (T lymphocytes). These cells directly attack other cells that have been infected with viruses and certain bacteria, thus they are effective against intracellular pathogens. There are types of T cells such as cytotoxic, helper, and suppressor T cells that play a role in the immune response by either destroying infected cells, aiding in the activation of other immune cells, or regulating the intensity of the immune response.
Natural killer cells (NK cells) and antigen-presenting cells (APCs) are important components of the immune response, but they do not partake in the adaptive immune response in the same manner as B and T cells. NK cells are part of the innate immune system and provide a nonspecific defense, while APCs present antigens to T and B cells to facilitate their activation.